The present invention relates to multiplex carrier frequency scanning control in optical frequency division multiplex communications.
In an optical communication network which transmits and receives optical frequency (waveform) multiplexed carriers it is necessary, for selecting each carrier to receive correct information, that stabilization control be effected at the transmitting side to keep predetermined spacing or intervals of multiplexed carrier frequencies (waveforms) to suppress interference between the carriers.
There has been proposed an optical frequency multiplexed carrier control system which employs an optical frequency standard light source having an optical frequency controlled for stabilization by molecular or atomic absorption lines so as to detect frequency variations of multiplexed carriers. (ELECTRONICS LETTERS, 5th January 1989, Vol. 25, No. 1, pp.9-11).
FIG. 4 shows the construction of a conventional optical frequency multiplexed carrier control system. Optical signals from transmitters are multiplexed (N multiplex channels) optical frequency multiplexer 1 together with an optical frequency standard and the multiplexed optical signals are introduced to an optical frequency spectrum detector, wherein it is incident to a scanning type Fabry-Perot means 4a and transmitted lightwave 4d therefrom is received by a photodetector 4b to detect the optical frequency spectrum of optical frequency standard and each carrier.
In this instance, since the optical frequency scanning region of the scanning type Fabry-Perot means 4a varies with an ambient temperature change, it is necessary to control each multiplexed carrier frequency while at the same time controlling the optical frequency scanning region relative to the optical frequency standard so that the optical frequency scanning region always maintains a preset optical frequency region of the multiplexed carrier spectra. To this end, optical frequency spectral waveforms of the optical frequency standard and the respective carriers are sampled by an optical frequency spectral waveform sampler formed by an A/D converter which operates in synchronization with a scanning signal and the sampled outputs are provided to spectrum sampling control, optical frequency spectrum interval detector and control information generator 6 formed by a computer for control use, wherein variations in the scanning optical frequency spectrum region and the respective carrier spectra with respect to the optical frequency standard are detected on software. By effecting negative feedback control of a scanning signal generator 4c and light source (l through N) for multiplexed carriers through control signal generator 7 so that the above-mentioned variations may remain constant, the optical frequency multiplexed carriers can be controlled.
In the sampling of the optical frequency spectral waveforms of the optical frequency standard and the respective carriers through A/D conversion, the accuracy of the detection of the frequency spacing between the optical frequency standard and the respective carrier spectra is limited by the resolution of the A/D converter used, this reduces the accuracy of stabilization control of the carrier frequency with respect to the optical frequency standard. With enhanced resolution of the A/D converter, the frequency spacings detecting accuracy could be improved, but since the number N of pieces of data to be transferred is usually greater than 1000, the time for data transfer to the control computer would become so long that the control delay time would also be increased, making the entire system uncontrollable.